1. Field of the Invention
The present invention relates in general to spark plugs for internal combustion engines and more particularly to captive spark plug gaskets.
2. Description of the Related Art
A spark plug has a metal shell. The metal shell has a threaded portion for securing the spark plug to a cylinder head of an engine. The spark plug is generally provided with an annular gasket which is so-called a captive spark plug gasket. The gasket is generally formed from an annular metal sheet by bending the metal sheet radially thereof. The gasket is mounted on the metal shell at a location adjacent an inner end of the threaded portion. At a location adjacent the inner end of the threaded portion, the metal shell has a flanged, seat portion. When the threaded portion is screwed into a threaded hole of the cylinder head to cause the gasket to be compressed between the seat portion and the outer surface of the cylinder head around the threaded hole, the gasket provides a seal between the outer surface of the cylinder head around the threaded hole and the seat portion of the metal shell for thereby providing a seal between the threaded portion of the metal shell and the threaded hole.
In this connection, to attain a seal by means of the gasket, it is important to control the tightening torque for screwing the treaded portion into the threaded hole so that a proper compressive force is applied to the gasket. To this end, for each type of spark plug, a recommended tightening torque range for attaining a good seal is fixed. FIG. 14 shows a relation between tightening torque and compressive deformation xcex1 of gasket due to advance or axial movement of the threaded portion, which is exhibited by a prior art gasket. The prior art gasket heretofore used causes, at the initial state of deformation, the compressive force to concentrate at a bent portion or portions thereof and is thus compressed and deformed considerably even when the compressive force is still within a relatively low range and tightening of the spark plug is still at the initial stage. In such a low range, a tightening force necessary for attaining a desired seal can be hardly obtained.
However, when the deformation proceeds further to make smaller the space for allowing further deformation, a larger compressive force is necessitated for further compressive deformation, thus causing the tightening torque to increase abruptly. As shown in FIG. 14, in case of the prior art gasket, the recommended tightening torque range for the spark plug, which is indicated by the hatched area, is located on a sharply sloped line portion of the graph where the tightening torque increases sharply or abruptly with increase of deformation. From this, it will be understood that the gasket deformation xcex1 range capable of attaining the recommended tightening torque range is quite narrow.
In recent automotive engines, the lean air-fuel mixture is used increasingly as the restriction on the exhaust emission control becomes severer (i.e., so-called lean-burn engines are used increasingly). As shown in FIG. 13, since the lean mixture is low in the ratio of fuel mixed, misfire may possibly be caused in case the mixture is introduced into the combustion chamber K in a certain direction (or in case the intake valve is disposed in a certain position) to cause the flow of the mixture into the spark gap g between the center electrode 203 and the ground electrode 204 to be obstructed by the ground electrode 204. For this reason, this kind of engine generally takes instructions that the threaded portion 201a of the metal shell 201 of the spark plug 200 is screwed into the threaded hole S1 of the cylinder head SH so as to orient the ground electrode 204 to optimize ignition.
However, when the threaded portion 201a is screwed into the threaded hole S1 to such an extend as to attain a desired seal, the ground electrode 204 is not always oriented to optimize ignition. In this connection, if the threaded portion 201a is tightened further or loosened so as to adjust the orientation of the ground electrode 204, the tightening torque changes to be out of the above described recommended limits since the gasket deformation xcex1 range capable of attaining the recommended tightening torque is so narrow. For example, when further tightening causes the tightening torque to become larger beyond the upper limit of the recommended tightening torque range, there is a possibility that the gasket 206 and/or the threaded portion 201a of the metal shell 201 is damaged to deteriorate the seal, and in the worst case, part of the metal shell is twisted off from the remaining section. On the contrary, when the threaded portion 201a is loosened to cause the tightening torque to become smaller than the lower limit of the recommended range, a desired seal cannot be obtained, thus leading to a trouble of leakage of gas or the like. In the meantime, indicated by reference numeral 202 is an insulator.
It has been proposed, as for example disclosed in Japanese Patent Provisional Publication No. 11-13613, to enable the ground electrode 204 to be oriented to optimize ignition, even if the recommended torque range is so narrow, by making the positional relation between a starting end of the thread of the threaded portion 201a and the ground electrode 204 constant. However, this method requires positioning of the ground electrode 204 with respect to the starting end of the thread of the threaded portion 201a, thus requiring much labor and longer working time and therefore causing lowered manufacturing efficiency and increased cost.
It is accordingly an object of the present invention to provide a gasket for a threaded element such as a spark plug, which enables to adjust the orientation or circumferential location of a certain reference part of a threaded portion of the threaded element within wide limits, under the condition where a suitable tightening torque is maintained, and accordingly which enables to adjust the orientation of a ground electrode of a spark plug relative to a cylinder head with ease and without causing any problem.
It is a further object of the present invention to provide a spark plug or the like threaded element with a captive gasket of the foregoing character.
To accomplish the above objects, there is provided according to an aspect of the present invention a gasket for a threaded element to be screwed into a threaded hole of a support. The threaded element has a threaded portion and an annular seat portion. The gasket is adapted to be installed on the threaded element and compressed between the seat portion and an outer surface of the support around the threaded hole, when the threaded element is screwed into the threaded hole, to provide a seal between the threaded hole and the seat portion. The gasket is formed from an annular sheet material and in the form of an annular strip of a cross section including a plurality of bent portions. The cross section is made by a plane including a center axis of the gasket and of a such a bent shape as to enable an imaginary reference line which is located on the plane and parallel with the center axis, to cross at least three portions of the cross section. An initial axial size of the gasket is at least 2.5 mm.
The structure for enabling the reference line parallel to the center axis to cross at least two portions of the cross section, means that, when a gasket 70 shown in FIG. 2 is taken as an example and reference is also made to FIG. 12A, the cross section is so shaped as to extend from a portion Sg1 crossed by the reference line PB to another portion Sg2 crossed by the reference line PB by way of a turnaround portion DC1. When the gasket 70 is deformed yieldingly or plastically in the direction of the center axis O, its deformation is mainly caused by buckling of the above described turnaround portion DC1. The structure for enabling the reference line PB to cross at least three portions of the cross section, means that such a cross section has at least two turnaround portions. Based on the above structure, it was discovered by the applicants that to set the initial axial size (i.e., the height of the gasket before the gasket 70 is compressed and hereinafter also referred to simply as height) of the gasket 70 to at least 2.5 mm makes it possible to adjust the circumferential location (or angular position) of a certain reference part of the threaded portion within relatively wide limits, under the condition where a proper compressive or tightening force is held applied to the gasket 70. The present invention is made based on such a discovery.
The reason why the above described effect can be produced by the present invention is considered as follows. Firstly, at the initial stage of compression, the respective turnaround portions resiliently deformed so that the stress increases relatively sharply with increase of deformation (first stage). When the applied pressure reaches to a certain level, bucking of the turnaround portions accompanied by plastic deformation starts, thus causing the rate of increase of pressure relative to increase of deformation to become more gradual (second stage). According to the present invention, the height of the gasket is set to at least 2.5 mm. By this, the portions of the gasket, by which the above described buckling is mainly caused, have the total length larger than that of the prior art gasket and constituted by at least two turnaround portions which are disposed separately. Thus, the pressure produced at the second stage where the circumferential location of a certain reference part of the threaded portion can be adjusted within relatively wide limits, can be increased up to such a level as to enable a proper compressive or tightening force to be applied to the gasket.
In the meantime, when the height of the gasket is smaller than 2.5 mm, it is difficult to make adjustment of the circumferential location of a certain reference part of the threaded portion within wide limits. On the other hand, in case, for example, the gasket is applied to a spark plug, it is necessary to set the upper limit of the height of the gasket so that the gasket does not interfere with the circumferential periphery of the countersink of the support (i.e., cylinder head) when deformed yieldingly or plastically to increase in outer diameter. Although the upper limit value varies depending upon a variation of the width of the gasket before the gasket is deformed, it is preferable to set the height of the gasket within the range equal to or smaller than 4.5 mm. In the meantime, it is more preferable to set the height of the gasket within the range from 2.7 to 3.5 mm.
According to another aspect of the present invention, assuming that the pressure P applied to the gasket is expressed by P=F/{xcfx80(R12xe2x88x92R22)} where R1 is xc2xd of the initial outer diameter of the gasket, R2 is {fraction (1/2)} of the initial inner diameter of the gasket and F is a compressive force applied to the gasket, a variation xcex94xcex1 of compressive deformation xcex1 of the gasket in response to a variation of the pressure P within the range from 6 to 12 kgf/mm2 is at least 0.5 mm.
In the above structure, the pressure range wherein a proper compressive force is applied to the gasket (hereinafter referred to as a proper pressure range) is determined so as to be from 6 to 12 kgf/mm2. The gasket is constructed so that a variation xcex94xcex1 of compressive deformation xcex1 of the gasket in response to a variation of the pressure P within the range from 6 to 12 kgf/mm2 is at least 0.5 mm, whereby it becomes possible to adjust the orientation or circumferential location of a certain reference part of the threaded portion within relatively wide limits.
An example of the thread element which the gasket of this invention is particularly effectively used with, is a spark plug for an internal combustion engine such as an automotive engine. The spark plug includes a center electrode, an insulator surrounding the center electrode, a tubular metal shell mounted on the insulator and having a threaded portion, and a ground electrode connected to the shell and spaced apart from the center electrode so as to provide a spark gap therebetween. In this instance, the metal shell has the threaded portion and a seat portion on the outer periphery thereof.
When the gasket of this invention is applied to a spark plug, it can produce the following effects. As described above, for the purpose of improving the assuredness with which the air-fuel mixture is ignited by the spark plug, the orientation of a certain reference part of the threaded portion when screwing of the threaded portion into the threaded hole is finished, is instructed in many lean-burn engines so that the ground electrode is oriented to optimize ignition. In this instance, the gaskets usually vary in size and material to some extent and the positional relationship between the starting end of the thread of the threaded portion and the ground electrode is not constant. For this reason, there can possibly occur such a case in which when the threaded portion is screwed into the threaded hole until the pressure on the gasket (i.e., tightening torque) reaches a predetermined, target value, the ground electrode is not always oriented to optimize ignition but is largely displaced from the target circumferential location.
However, the gasket of the present invention is constructed so that a variation xcex94xcex1 of compressive deformation xcex1 of the gasket in response to a variation of the pressure P within the above described proper pressure range wherein a proper compressive force is applied to the gasket is at least 0.5 mm and the range of turn of the thread corresponding to the variation xcex94xcex1 is wide. As a result, it becomes possible to turn the threaded portion within relatively wide limits, under the condition where a proper compressive force on the gasket is retained, thus making it possible to attain desired adjustment of the circumferential location of a certain reference part of the threaded portion, i.e., desired adjustment of the orientation of the ground electrode without causing any problem. Thus, the gasket of this invention can dispense with the control of the positional relationship between the starting end of the thread of the threaded portion and the ground electrode, so there is not any possibility of causing such lowered production efficiency as is caused by the prior art disclosed in Japanese Patent Provisional Publication No. 11-13613.
In this instance, it is preferable that the variation xcex94xcex1 of compressive deformation xcex1 of the gasket in response to a variation of the above described proper pressure range wherein the pressure P on the gasket ranges from 6 to 12 kgf/mm2 is of the length corresponding to at least 0.5 pitch of the thread of the threaded portion, i.e., of such a length as to allow the threaded portion to advance or axially move at least 0.5 pitch. This means that the threaded potion can be rotated by at least 0.5 turn within the pressure range in which a proper compressive force is applied to the gasket. Accordingly, adjustment of the circumferential location of a certain reference part of the threaded portion relative to the cylinder head can be made by rotating the threaded portion by 0.5 turn or more, i.e., within wide limits. In the meantime, the variation xcex94xcex1 of compressive deformation xcex1 is preferably of the length corresponding to at least one-pitch of the thread.